The Nobel Nobel prize
for chemistry for 1928 was awarded to Adolf
Windaus "for his studies on the constitution of the sterols and
their connection with vitamins". The vitamin in question was vitamin
D and Windaus was the first scientist to receive an award mentioning vitamins.
A comprehensive review of the vitamin D history viewed through the contribution
of Windaus may be found in a paper by Wolf G (J Nutr 2004, 134, 1299).

Vitamin D2 (calciferol) originates from irradiation at about 260nm of ergosterol
while vitamin D3 is formed by irradiation of a provitamin molecule (7-dehydrocholesterol)
present in the skin and gut lining. From a physiological point of view, Steenbock
H et al. discovered the formation of an anti-rachitic factor in rats under UV
irradiation (J Biol Chem 1924, 61, 405). Vitamin D3 was isolated around
1936 (Brockmann) from the very potent liver oil of the tuna fish. The unsaponifiable
fraction was partitioned between 90% methanol and petroleum ether to separate
vitamins A and D. The residue from the petroleum phase contained vitamin D3
to be purified by multiple chromatography. The total synthesis was done around
1952 (Woodward, J Amer Chem Soc, 74, 4223). When tested on rats, the
two vitamins are equally potent (vitamin D3 is more active than D2 in the chick
test). The international unit has been so defined as 40 i.u./µg. Natural fish
liver oils vary from about 100 i.u./g in cod to 200,000 i.u./g in tuna fish.
DeLucaHF
(Holick M F et al., Biochemistry 1971, 10, 2799), Norman AW (Biochem
Biophys Res Commun 1971,42, 1082), and Kodicek E (Fraser DR et
al., Nature 1970,Nature228, 764) independently reported
the existence of an active metabolite, 1,25-dihydroxyvitamin D3, which was produced
in the kidneys. From these observations it was surmised that vitamin D3 was
hydroxylated in the liver to become 25-hydroxyvitamin D3, the major circulating
form of the vitamin, and then converted to 1,25-dihydroxyvitamin D3 in the kidneys.
Thus, this final metabolic product is now considered as the metabolically active
form of vitamin D3, which carried out its functions in initiating intestinal
calcium transport. As the generation of vitamin D3 by UV-mediated photosynthesis
is the main source of this secosteroid compound, it can be considered that vitamin
D3 is not a true vitamin but a true hormone.

It has been shown that vitamin D3 is present in plants, especially in species
belonging to the family Solanaceae. The first demonstration that Solanum
glaucophyllum living in South America is a source of 1,25-dihydroxyvitamin
D3 has been done in 1977 (Napoli
JL et al., J Biol Chem 1977, 252, 2580). The concentration was estimated
to 230 pg/g of tissue. Later, it was demonstrate that grass potentially can
be a significant source of vitamin D2 (average content was 2ng/g fresh weight)
for grazing animals and animals fed on silage and hay (Japelt RB et al.,
J Agric Food Chem 2011, 59,10907). Vitamin D3 and its hydroxylated metabolites
were isolated from leaves of Solanum glaucophyllum but also of tomato
(S. lycopersicum) and bell pepper (Capsicum annuum) (Japelt
RB et al., Food Chemistry 2013, 138, 1206). Vitamin D3 and 25-hydroxy vitamin
D3 were found in the leaves of all plants after UVB-treatment. S. glaucophyllum
had the highest content, 200 ng vitamin D3/g dry weight and 31 ng 25-hydroxy
vitamin D3/g dry weight, and was the only plant that also contained 1,25 dihydroxy
vitamin D3 in both free (32 ng/g dry weight) and glycosylated form (17 ng/g
dry weight).

The foods that are the richest in vitamin D are fatty fish (5-20 microg/100
g) and eggs, with smaler amounts being present iin milk, butter, and some meats.
The concentrations in several foods may be found on the French CIQUAL
table and on the Australian and New Zealand online searchable NUTTAB
data base.

It is now well known that vitamin D is a central player in calcium and bone
metabolism, but more recently, important immunomodulatory effects have been
attributed to this vitamin (Baeke
F et al., Mol Aspects Med 2008, 29, 376). Vitamin D deficiency/insufficiency
has been associated with muscle weakness and a high incidence of various chronic
diseases such as cardiovascular disease, cancer, multiple sclerosis, and type
1 and 2 diabetes (Zittermann A et al., Nutrients 2010, 2, 408). Most
importantly, low vitamin D status has been found to be an independent predictor
of all-cause mortality (Zittermann
A et al., Curr Opin Clin Nutr Metab Care 2009, 12, 634).
A review of the new trends in the physiological actions of vitamin D has been
released in 2009 (Dixon
KM et al., Int J Biochem Cell Biol 2009, 41, 982) and may be found in
my book (Lipids
- Nutrition and health, Claude Leray, CRC Press).

Initially, the determination of vitamin D was based on a reaction between antimony
trichloride giving a yellow color with l max 500nm.Removal of vitamin A and
E was necessary. Partition chromatography is actually used with success.

vitamin D2

Vitamin D3

Molecular weight

396.6

384.6

l max

265

265

Molar
extinction coefficient

18,200

18,200

m.p.

116°C

83°C

For humans, the requirements are about 100
i.u./day for a man but 400 i.u./day for a child or a pregnant woman.

The analysis of the vitamin D
metabolites is dominated by immunoassays and receptor binding assays but an
efficient profiling of major vitamin D metabolites has been described using a
Diels-Alder derivatization and ultra-performance liquid chromatography-tandem
mass spectrometry (Aronov PA et al., Anal Bioanal Chem 2008, 391, 1917).
A review of that approach has been reviewed (Higashi
T et al., J Chromatogr B 2010,878, 1654).